(1) Field of the Invention
The present invention relates to a melt molding method of germanium, and particularly to melt molding of a germanium lens and the like which are useful for an infrared lens and the like.
(2) Description of Related Art
Conventionally, in JP-A-63-157754 for example, a germanium lens for infrared measurement is produced by increasing the temperature of a germanium raw material to not lower than a melting point, casting the liquid germanium into a mirror-finished lens casting mold, and cooling the casting mold. Further, in this method, a nitrogen gas atmosphere is adopted in order to prevent entry of impurities, and further, sealed nitrogen gas is evacuated to produce a vacuum, and air and the like are defoamed from the germanium liquid. Thereby, the germanium lens is molded into a required shape at a time.
However, unlike other metals and glass, germanium has the problem of expansion in volume when germanium is solidified, causing cracks, bulges, and depressions. Thus, in JP-A-7-314123, melt molding is performed by a casting mold by injecting a germanium molten liquid at a high pressure into a casting mold, cooling the germanium molten liquid while enhancing the density, decreasing the injection pressure at a temperature close to a solidifying point to absorb pressure caused by solidifying expansion of the material and prevent occurrence of internal distortion, and enhancing the injection pressure again at a temperature not higher than the solidifying point. Further, temperature control is performed by measuring the temperature of a molding die and the temperature inside a heating furnace with a temperature monitor. Further, a gas supply tube is provided at a lower portion of the molding die, and reducing gas is supplied to substitute for moisture and the like in raw material powder.